Beaconsfield Dam is an asset owned and maintained by Melbourne Water, with the land surrounding the reservoir managed by the Department of Environment Land Water and Planning (DELWP). The reservoir was permanently disconnected from Melbourne's water supply and distribution network in 1988. The priority for the Beaconsfield Dam Safety Upgrade project is to protect properties and communities located downstream of the reservoir.
Why is this project important?
Beaconsfield Dam was built over 100 years ago and needs to be upgraded in order to meet current guidelines set by the Australian National Committee on Large Dams (ANCOLD). Melbourne Water has a strong emphasis on dam safety management principles set out by these guidelines. These guidelines apply to large and small dams that could present a risk to communities downstream.
To determine the best way to improve safety, four possible options were assessed.
- Option one: Leave water level at current height and continue to monitor
- Option two: Full decommission of the reservoir
- Option three: Undertake a safety upgrade, retain existing water level and increase the height of the embankment
- Option four: Partial decommissioning of the reservoir. Reduction of the height of the embankment and lowering the water level
Melbourne Water led a Multi Criteria Analysis (MCA) in 2018 involving stakeholders and some members of the community against all four options.
Over the past 12 months, Melbourne Water has undertaken additional assessments against the four criteria, including engaging independent ecologists to undertake environmental and conservation assessments and impact on lowering the water level.
Option four (partial decommissioning) is the preferred and agreed option. This is based on the assessments conducted and it would successfully reduce potential consequences whilst still maintaining a permanent water body. This option is also a cost effective approach and reduced impacts on the environment that can be managed.
What does this option involve?
Partial decommissioning of the reservoir will involve:
- Lowering the dam crest from 98.5m to 94.00m – reducing the peak water level and modifying the current low-level outlet to act as the new water discharge point.
- Removal of infrastructure that will no longer be needed
- Constructing a new energy dissipater to reduce scouring and safely discharge into Haunted Gully Creek, and building a 10m long secondary rock-lined spillway are all part of the safety works.
Protecting the Environment
In June 2021 Arthur Rylah Institute (ARI) for Environmental Research conducted a desktop assessment of environmental values and potential impacts. This report focused on the lowering of the water level and how that would impact the surrounding environment.
The ecological report recommends a slow draw-down of the water level over a minimum three-year period to allow the vegetation around the edge of the reservoir to migrate with the changing waterline. This will minimise the impact on the environment. Melbourne Water will commence lowering the water level in 2022.
- What are ANCOLD Guidelines and how do they relate to Beaconsfield Reservoir?
Under the Water Act 1989, Melbourne Water is responsible for the safety of our dams, including Beaconsfield Reservoir.
We reduce risks by conducting routine dam surveillance, inspections and detailed risk assessments, as part of our proactive approach to managing dams.
The management of all our dams is in line with the ANCOLD guidelines, which represent the best Australian and international engineering practice in the safe design, management and operation of dams. They provide a consistent framework for assessing risk and allow Melbourne Water to prioritise safety upgrade works across our portfolio of dams.
In Victoria, the ANCOLD guidelines are further underpinned by state dam safety regulations in Melbourne Water’s Statement of Obligations from the DELWP – the Dam Safety Regulator for Victoria.
The Beaconsfield Dam Safety Upgrade project is part of our ongoing Dam Upgrade Program to ensure continued compliance with the ANCOLD guidelines into the future. Under this program, we have completed upgrades to the Frankston, Tarago, Toorourrong, Greenvale and Upper Yarra Reservoirs and soon to commence, Cardinia Reservoir.
The Beaconsfield Dam, in its current state, does not comply with ANCOLD guidelines. Proposed upgrade works will ensure it meets requirements in line with current industry standards and ensure protection of the local community who live downstream of the dam.
- What information was provided in the Environmental Assessment?
The ecological report recommends a slow draw-down of the water level over a minimum three-year period to allow the vegetation around the edge of the reservoir to migrate with the changing waterline. The lowering of the water level will commence in 2022.
The report also outlined recommendations to minimise the risk to plants and wildlife in and around the reservoir.
This includes conducting a detailed ecological assessment prior to commencing any upgrade works, monitoring the impacts to biodiversity, controlling woody weeds and collecting seed from Swamp Gum and Green Scentbark in the first year of lowering the water level.
Following the recommendations from the desktop ecological assessment, ARI will undertake formal flora, waterbird and frog surveys and also targeted surveys for species that are known or likely to be present within the reservoir. Particular focus will be on Growling Grass Frog, Swamp Skink, and Southern Toadlet.
Field assessments have already begun to identify and collate a list of known flora directly reliant on the reservoir. These field assessments will continue seasonally over the next 12 months.
Following the recommendations from ARI, on the dam will not begin until later 2024 or 2025 in order to allow the slow draw down of the water level.
- Has there been any consultation with the Traditional Owners?
Melbourne Water has held an initial project briefing with the Bunurong Land Council Aboriginal Corporation (BLCAC). Further engagement to develop a Cultural Heritage Management Plan will occur when we are closer to planning the upgrade.
- What is Melbourne Water’s rationale for undertaking the proposed Dam Safety Works?
The dam was built over 100 years ago and does not meet current Australian National Committee on Large Dams (ANCOLD) guidelines.
Retaining the dam in its current state, would not comply with these ANCOLD guidelines.
As a minimum, dam managers are required to achieve a level of dam safety which is tolerable and where this is not the case, undertake further measures to reduce the risk.
Australia has a strong emphasis on dam safety management principles set out by these guidelines.
Ensuring all Melbourne Water's dam meet modern safety standards is the main driver of the Beaconsfield Dam Safety Upgrade project.
While the likelihood of dam failure is low, the consequence would be significant.
- What is the data and/or evidence to support that the reservoir’s wall does not meet guidelines.
Beaconsfield Dam embankment, spillway and embankment performance does not meet modern/contemporary standards during flood events. In light of our knowledge of these shortcomings, Melbourne Water has a duty of care to upgrade the structure. We understand that the reservoir water body offers amenity value to the community and this has been carefully considered throughout the decision-making and engagement process.
- GHD investigation in 1999 observed left side of embankment needed upgrading.
- 2010 detailed risk assessment by AECOM identified piping along the spillway interface and downstream of the embankment needed upgrading.
- Seepage through the embankment, most recently observed in August 2018 on the downstream right abutment groin needed to be addressed.
- Why is the reservoir not suitable for use as a potable water reservoir?
Beaconsfield Dam has not been connected to the water distribution network since 1988. Cardinia Reservoir now provides a safe and secure water supply to the Mornington Peninsula.
The dam has a very small catchment, and the reservoir water level stays relatively constant, which indicates that losses due to evaporation match inflows into the dam in most years. If water was incorporated into the potable system, there is a low likelihood that the dam would be replenished.
Historically the reservoir was filled via an open channel aqueduct (Tarago Main Race) from Tarago Reservoir, but due to water quality issues, the condition of the Tarago Main Race, and Cardinia Reservoir being brought online, it was abandoned.
- Will lowering of the water level limit firefighting capability?
Melbourne Water has received the following advice from DELWP’s Chief Fire Officer:
Beaconsfield Reservoir is not a pre-approved location for water pickup by firefighting aircraft. Pilots look for 70m clearance from nearest trees to reduce risk of an aircraft being caught in surrounding vegetation.
Whilst the site could potentially be used for firefighting purposes, nearby Lysterfield Lake, Aura Vale Lake and Cardinia Reservoir – are all designated and approved as water pickup locations.
Cardinia Reservoir is a pre-approved site, 6km to the north which is much larger and safer to fill from. Firefighting helicopters travel at 1.5 to 2km per minute and save 3 minutes filling from Beaconsfield.
- What community consultation has occurred to date?
September 2016: Melbourne Water (MW) initiated consultation with those with an active interest and responsibility at the site, primarily the Cardinia Environmental Coalition (CEC) and DELWP.
August 2017: Concept design of reducing dam height was shared at workshop with community (CEC, DELWP and Council).
February 2018: Follow up workshop with community (CEC and Friends of BNCR).
June 2018: MW present at public meeting with Upper Beaconsfield Association.
July 2018: MW host open day within the site for the community.
October 2018: MW met with concerned community members to share technical details of the project.
March 2019: MW present to community (CEC and Friends of BNCR) with a revised concept.
April 2019: six small community sessions held in Upper Beaconsfield and Officer to discuss project and the concept. Digital engagement occurred simultaneously with 30 responses received.
- Will there be any further community engagement opportunities?
Melbourne Water will look to run a number of information drop in sessions in early 2022. We will provide details on the project website; melbournewater.com.au/Beaconsfield-dam once they have been scheduled.
- When will work on the dam begin?
Melbourne Water will be following the advice from ARI and lower the water level down slowly to allow the environment the best chance to establish. We will continue to regularly monitor the embankment with a view to commencing the works in 2025.
About Beaconsfield Reservoir
The Beaconsfield Dam sits within the Beaconsfield Nature Conservation Reserve (BNCR) on the Haunted Gully Creek, located within Cardinia Shire, approximately 45km south-east of Melbourne. The reservoir is on-stream storage, with a local catchment area of around 334ha.
It was constructed by the State Rivers and Water Supply Commission in 1918 as part of the water supply scheme for the Mornington Peninsula. Water was harvested from the Bunyip River and conveyed to Beaconsfield Reservoir by the Bunyip Main Race which was later supplemented by the construction of the Tarago Main Race.
The reservoir was permanently disconnected from Melbourne's water supply and distribution network in 1988, following the connection of Cardinia Reservoir.
Beaconsfield Reservoir is owned by Melbourne Water, but is located on Crown Land managed by the Department of Environment Land Water and Planning (DELWP). The Cardinia Environmental Coalition (CEC) manage the surrounding Beaconsfield Nature Conservation Reserve (BNCR) under an agreement with the Minister for Water.
About Beaconsfield Nature Conservation Reserve (BNCR)
In 2005, the state government gazetted the 172-hectares surrounding the reservoir be set aside as the BNCR to help conserve species of plants and animals that may be rare or endangered, contain critical habitat, or hold conservation significance. The BNCR includes a variety of vegetation listed as endangered, vulnerable, and depleted in Victoria's east.